By Blaise Jones
Sharks are amazingly resilient to disease. Research has indicated that sharks may be the first vertebrate animals on our planet to have developed the modern immune system and thus have had hundreds of millions of years to perfect it in ways that are better than human resistance to diseases.
Like other vertebrates including humans, sharks have an immune system that relies on many ingredients such as immunoglobulin, T-cell receptors, and lymphoid tissues that work together to fight off disease and infection in the body. However, sharks do not have any bones, just a cartilaginous skeleton attached to a spinal cord. In humans and other vertebrate mammals, important immune cells called lymphocytes are created and deployed from bone marrow. Since sharks do not have bones, they do not have bone marrow. Nor do they have lymph nodes. Instead sharks have unique lymphocyte producing tissues found in their esophagus and their gonads. Consequently, immune cells are always present in shark blood, whereas in other vertebrates (like humans) these cells are released in response to sickness.
In other words, the blood of sharks is always ready to fight off any infection, unlike the blood of humans and other vertebrates that only deploys infection-fighting compounds after sustaining injury or infection.
While the immune system of sharks is highly efficient, it’s not perfect. Sharks get sick just like every other animal on the planet.
Diseases are caused by a wide variety of sources: viruses, bacteria, pollutants, parasite, fungi, and even algae. Sharks have very good defense mechanisms, but they are not immune from every possible contagion. Some common diseases found in sharks include viral dermatitis (a type of herpes), several types of Vibro bacteria (known to cause infections), Staphylococcus, and Pasteurella.
In fact, some sharks are especially vulnerable to certain diseases. Fusarium solani is a type of fungus so common in bonnethead hammerhead sharks that it is known as the “bonnethead shark disease.” The fungus has been found in all species of hammerhead shark as well as in zebra sharks.
Many of these sicknesses are caused by stress. Changes in water temperature, pH, and the addition of pollutants and chemicals have been noted to cause immunosuppression in sharks, leading to increased susceptibility to parasites and diseases.
Cancerous tumors have been found in sharks for more than a century, the first being described in 1908. Since then tumors have been identified in more than 18 shark species. However, any cancer-specific research on sharks is difficult because the infected sharks often die before researchers are able to locate the individuals or fisherman simply discard the sick sharks since they cannot be sold.
So why does the myth that sharks are immune to cancer persist?
The shark cancer myth
The myth that sharks cannot contract cancer started in the 1970s, when shark research was in its infancy. Henry Brem and Judah Folkman of the John Hopkins School of Medicine discovered that cartilage prevented the growth of new blood cells in tissue. As this blood vessel growth is a key part of the growth of cancerous tumors, the two speculated that perhaps cartilage could be used to treat cancer. Another researcher, Robert Langer, reasoned that sharks would be an easy source of cartilage and found that treating tumors with shark cartilage prevented blood vessel growth. Using this research, Carl Luer of Mote Marine Laboratories tested shark susceptibility to cancer by treating sharks with high levels of aflatoxin B1, a known carcinogen. Luer claimed that none of his sharks developed cancer.
Many medical charlatans used Luer’s findings to claim that they had created medicine using shark cartilage to cure or prevent cancer. One noted fraud was Dr. I. William Lee – who is not a medical doctor, but rather earned a Ph.D. at Rutgers University for biochemistry – claimed such ridiculous medical positive results from his company’s shark pills that in 2004 the United States government banned his company’s false advertisements.
Though Lee’s company may not profit in the U.S., the myth of shark cartilage helping to cure cancer spread throughout the world, especially in markets where governments didn’t block the false advertising that spread the rumors.
So now that we know that sharks can get sick, the question is can we get sick from sharks? Unfortunately, sharks are carriers for many different zoonotic diseases (ones that can pass between humans and animals).
The most commonly disease humans can catch from sharks is Giardia, a parasite that infects the intestines. It isn’t fatal, usually results in diarrhea, and is passed out of the body typically in two to six weeks.
Other diseases humans can catch from sharks include Acinetobacter calcoaceticus-baumannii, Citrobacter braaki, Citrobacter freundii, Enterobacter cloacae, Leclercia adecarboxylata, Morganella morganii, Pseudomonas aeruginosa, Pseudomonas spp., Shewanella spp. and Stenotrophomonas maltophilia. Each of these diseases can be dangerous to humans, with side effects including blood-poisoning, infection, pneumonia, and sepsis.
How does a human get a shark-borne disease? By being bitten by a shark. While shark bites are very rare, one of the chief concerns that rise from them is infection. Like with humans, the most infected place on the body for a shark is his mouth. Consequently, even though a shark bite may be small and non-threatening, the diseases present on their teeth and in their mouth can cause a great deal of long-term damage even after the would heals. Which is yet another reason to avoid getting bitten by a shark.
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